1. Field of the Disclosure
The present invention relates generally to an apparatus and method for regulating an additional output of a power converter.
2. Background
Many electrical devices such as cell phones, personal digital assistants (PDA's), laptops, etc. are powered by a source of dc power. Because power is generally delivered through a wall outlet as high-voltage ac power, a device, typically referred to as a power converter or power supply, is required to transform the high-voltage ac power to low-voltage dc power for many electrical devices. In operation, a power converter may use a controller to regulate output power delivered to an electrical device that may be generally referred to as a load. In operation, a controller may control a transfer of energy pulses created by switching a power switch on and off in response to feedback information. In one instance, the feedback information may be used to keep the output voltage at the output of the power converter regulated.
In certain applications, power converters may include an energy transfer element to separate an input side from an output side of the power converter. More specifically, the energy transfer element may provide galvanic isolation which prevents dc current from flowing between the input side (where power is provided) and the output side (where power is delivered) of the power converter and may be required to meet certain safety regulations.
In one example, an isolated power converter may be required to provide additional outputs with a regulated voltage. For example, an isolated power converter used for a computer may require a first regulated output at 5 V and an additional regulated output at 3.3 V. A common method to provide for an additional regulated output voltage is to include a post regulator. More specifically, a post regulator is coupled to the output side of the power converter to redirect energy from a regulated output in order to regulate an additional output. In one example, a post regulator may include a post regulator controller coupled to the output side of the power converter which may switch a switch coupled to the output side of the power converter such that a portion of energy delivered to the main output is redirected to an the additional output. The post regulator may control the switching of the post regulator switch to regulate the voltage at the additional output.
In one instance, the post regulator switch that regulates an additional output is implemented as a transistor, and to be more specific, is implemented as a metal oxide semiconductor field effect transistor (MOSFET) to accommodate for practical considerations such as performance and cost. When using a MOSFET to implement the post regulator switch, a substantial voltage may be needed to switch the MOSFET. However, the substantial voltage needed for the MOSFET to switch may not be available at the output side of the power converter. In the case of the example power converter for a computer mentioned above, the substantial voltage to switch the MOSFET may need to be 15 V, and the main output voltage is only at 5V. Thus, additional costs may be incurred for including multiple components for generating an adequate voltage in order to switch the MOSFET of the post regulator.